Bar Crossing on a Falling Tide with a Marginal Under-Keel Clearance
MV Anvil Point, a 38,000 dwt bulk carrier, is inbound to a river port with drafts forward 11.25 m and aft 11.45 m. The 12-mile approach has a least charted depth of 9.8 m below chart datum at the bar, graded CATZOC B, and carries a 0.4 m shoal patch last surveyed in 1998. Predicted tides at the bar are HW 0340, 2.9 m and LW 0950, 0.4 m. Company UKC policy is 10% of maximum draft in exposed waters and 5% in sheltered waters; the bar is classified exposed. Squat at the planned 8 knots through the 200 m wide channel is calculated at 0.85 m. River density is expected to be 1.015 t/m3 against 1.025 in the approach. The pilot boards at 0300 and wants to cross at 0430 to reach the berth on the tide; the berth window closes about 12 hours later and there is no safe anchorage inside the bar. Only one berth is available in the next five days.
1. Calculate the under-keel clearance that would exist over the bar and over the shoal patch at 0430, and state with reasons whether you would cross.
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Tide at 0430. HW 0340 is 2.9 m, LW 0950 is 0.4 m: a range of 2.5 m over 6 h 10 min. Fifty minutes after HW the fall is roughly 1/12 of the range, about 0.2 m, so the tide at 0430 is about 2.7 m. Predictions are not guarantees - wind set-up and river flow can move this by 0.2 m either way.
Depth available. Over the 9.8 m charted depth: 9.8 + 2.7 = 12.5 m. Over the 0.4 m shoal patch: 12.1 m.
Depth required. Draught 11.45 + squat 0.85 + 10% UKC 1.145 = 13.445 m. Density adds 11.45 x (1.025/1.015 - 1) = 0.11 m, so 13.56 m is needed.
Conclusion. The deficit is about 1.1 m over the bar and 1.5 m over the patch - aground, not marginal. I would not cross. Waiting for the next HW does not cure it either: 2.9 m of tide gives 12.7 m, still 0.9 m short. A lawful crossing needs roughly 4.1 m of tide over the patch (above MHWS) and a reduced draught of 0.6-1.0 m, which means several thousand tonnes lightened. The realistic plan is to lighten at the outer anchorage or hold for a spring tide with a verified, reduced arrival draught, and to re-run the calculation with the actual density and any measured tide.
2. The pilot says the charted depth is 'conservative' and that CATZOC B only means the position is uncertain by 20 m. How do you set safety depth and safety contour in the ECDIS, and what is wrong with treating charted depth plus tide as a UKC calculation?
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The pilot's statement misstates the category. CATZOC B is roughly +/-50 m position and sounding accuracy of +/-(1.0 + 2% of depth), about +/-1.2 m here, before you allow for a 1998 patch. 'Conservative' is an unquantified opinion; the number on the ENC already includes it or does not, and you cannot tell which.
Safety depth is the depth the ship must never pass through: maximum draught + required UKC, with squat, heel and density already inside the UKC figure - here 11.45 + 1.15 + 0.85 = 13.45 m. Adjust for tide only if the ENC is tide-corrected or you apply a manual allowance; the soundings themselves are referenced to chart datum. Safety contour is the nearest available ENC contour at or deeper than the safety depth, so that everything shoaler is isolated as unsafe; if no contour matches exactly, pick the next deeper one and know that the gap exists. Also set the shallow contour (draught plus squat), switch on the CATZOC display, read M_QUAL and M_SREL, and use look-ahead at a scale where the bar is visible.
The error in 'charted depth plus tide' is that it answers the wrong question: it gives water depth, not clearance. It omits squat, heel, density, tide prediction error, swell-induced heave on the bar and survey uncertainty. UKC must be a number you have budgeted for, not the arithmetic leftover.
3. How do you handle the commercial pressure from the pilot, agent and charterers, and what do you document?
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The decision is the master's, and compulsory pilotage does not transfer command - the pilot is an adviser. Under SOLAS V/34 and ISM 5.2 the master has overriding authority on safety, and no berth window, fuel budget or charter party obligation outranks a grounding risk that the calculation has already quantified at over a metre.
Practical sequence: (1) state the decision plainly to the pilot at the time, with the figures, and ask him to confirm he has heard it; (2) ask the harbour master for the latest survey of the bar and the official declared depth and density, since the charted figure may lag a dredging campaign; (3) notify the company, the DPA, the agent and the charterers in writing of the calculated shortfall, the options (lightening at the outer anchorage, waiting for a suitable spring tide, or proceeding to an alternative port) and the time implications; (4) request a lightening plan with tonnages and a revised draught, and a new berth slot; (5) record everything in the official log, the bridge notebook and the master's statement, and note protest if a party attempts to insist.
Re-run and file the calculation after lightening, using measured density and the day's actual tide, and set a firm go/no-go depth at the bar that the bridge team understands. Pressure recorded and answered in writing at the time is the master's best protection afterwards; an informal verbal discussion is not.